Null Pointer Exception

[ScreamingHawk]

I have a basic serverless program but deployment is failing with your plugin. Looks like the stderr is null.

C:\Users\Michael\serverless\node_modules\serverless-coffeescript\index.js:48
        const sterr = result.stderr.toString();
                                   ^

TypeError: Cannot read property 'toString' of null
    at Extract.stream.on (C:\Users\Michael\serverless\node_modules\serverless-coffeescript\index.js:48:36)
    at emitNone (events.js:110:20)
    at Extract.emit (events.js:207:7)
    at finishMaybe (_stream_writable.js:603:14)
    at endWritable (_stream_writable.js:611:3)
    at Extract.Writable.end (_stream_writable.js:562:5)
    at ReadStream.onend (_stream_readable.js:598:10)
    at Object.onceWrapper (events.js:314:30)
    at emitNone (events.js:110:20)
    at ReadStream.emit (events.js:207:7)
    at endReadableNT (_stream_readable.js:1059:12)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickDomainCallback (internal/process/next_tick.js:218:9)

Wrapping the line in a null check seems to do the trick. I'll make a pull request.